In order to make it impossible, for example, for a hose-type sprayer to take in any unclean water if its head it left immersed after the water supply is shut off, it is known from U.S. Pat. No. 4,827,538 to provide the mixing valve with a vent fitting that admits air to the mixing chamber of the valve. Such a fitting is provided in a passage extending between the mixing chamber and the surroundings and basically comprises a very sensitive check valve that permits flow inward into the valve when the pressure inside the valve is slightly less than the outside pressure, as low as 3 cm to 5 cm water column. Thus when the water is shut off the siphon effect of the water in the faucet or hose will cause this vent valve to open so that air is admitted to the mixing chamber and the fitting can drain. During normal use when the mixing chamber is pressurized, the vent valve is tightly closed. Thus if some underpressure is created in the supply lines, dirty water is not sucked back into the valve.
This arrangement has the disadvantage that it increases the size of the mixing valve, which normally is mounted atop the deck next to the sink or basin. Since the appearance of this item is critical, anything that increases its size constitutes a severe design problem.
A plumbing fitting is described in German patent document 3,805,462 filed 22 Feb. 1988 by W. Gnauert and published 31 Aug. 1989. It has a pressurizable chamber and is formed with a vent passage opening at an inner end into the chamber and at an outer end to ambient-pressure surroundings. A vent assembly comprises an inner check valve in the passage having an inner side exposed to pressure in the chamber and an outer side turned away therefrom and an outer check valve in the passage between the inner valve and the outer passage end and having an outer side exposed to the ambient pressure of the surroundings and an inner side turned away therefrom and forming with the passage and outer side of the inner valve a normally closed compartment. Each valve opens when pressure on its outer side exceeds that on its inner side and closes when pressure on its inner side exceeds that on its outer side.
Another such arrangement is described in German patent document 3,708,169 of K. Gute wherein a backflow preventer has a housing formed with an inlet port connected to pressurizable supply line, an outlet port connected to the outlet device serviced by the system, and a vent port. An accordion-like extension of the inlet port can reach into the outlet port when pressurized to connect the inlet and outlet ports together and cut out the vent port, but when depressurized it breaks the direct connection between the inlet and outlet ports and permits the inlet port to draw in through the vent port.
Both such systems can be located out of sight, but have a limited effectiveness but are susceptible of improvement.